The present invention relates to a circuit board used for electronic equipment, more specifically, it relates to a circuit board having an interstitial inner via hole structure, a method for manufacturing the same, and a circuit board electrically insulating material used therefor.
In recent years, as miniaturization, light weight and high performance of electronic equipment have been required, there has been a strong demand not only for a small size and light weight of a circuit board but also for higher speed for processing signals and high density mounting, and the like. In order to meet such demands, in the field of a circuit board technology, a technology for increasing the number of layers in a laminate structure, a technology of making the diameter of an inner via hole smaller, a technology for making circuit patterns finer, and the like have been developed rapidly, and circuit boards having various structures have been proposed or put into practice. As one example, the present assignee has developed a circuit board having an all-layer IVH (interstitial inner via hole) structure, in which the interlayer electric connection is secured with an inner via hole filled with a conductive paste in place of a through hole structure that has been a usual method in conventional circuit boards (see, for example, JP7 (1995)-14746 4 A). This circuit board has features that no through hole is needed and inner via holes can be arranged at arbitrary positions, and therefore, this circuit board has a high capacity of wiring, a high degree of freedom in design, shortening of wiring, etc. Thus, this circuit board is suitable for signal processing at high speed and high density mounting. In this circuit board, the conductive paste filled in the through hole is compressed and hardened simultaneously with molding of the laminate, thereby providing an inner via hole for securing an interlayer electrical connection. Therefore, this circuit board needs an electrically insulating material having such a compressive performance that permits sufficient compression of a conductive paste and is capable of maintaining the shape of the inner via hole. In general, as a conductive paste, a prepreg obtained by impregnating an aramid non-woven fabric with a thermosetting resin is used. The compressive performance of the aramid prepreg can be achieved by the melt flow of the thermosetting resin and the effect of defoaming of any remaining air foam. The maintenance of the shape of the inner via hole can be achieved by rigid property of the aramid non-woven fabric.
In the future, in a view that a circuit board will increasingly be required to have a finer circuit pattern, a smaller diameter of an inner via hole and an increase in number of the layers of the laminate structure and the like, it is extremely important to make the electrically insulating layer to be homogenized and ultra-thin. However, a non-woven fabric made of an aramid staple fiber having a diameter of 10-20 xcexcm may cause a non-negligible non-uniformity of the physical property in a micro area depending on the direction of the fiber. Furthermore, a satisfactory thin layer cannot be obtained because there is a limitation to the number of fibers in the thickness direction.
With the foregoing in mind, it is an object of the present invention to provide a circuit board electrically insulating material and a circuit board, in which an electrically insulating layer is made to be homogenized and ultra-thin and the connection reliability of the inner via hole including a conductive paste is improved so as to achieve higher density mounting, and also provide a method for manufacturing the same.
In order to achieve the above-mentioned objectives, the circuit board electrically insulating material of the present invention is a circuit board electrically insulating material in sheet form, insulating sheet, including a porous sheet; and a resin layer laminated to a surface of the porous sheet, wherein at least a central portion of the porous sheet is not impregnated with resin and the circuit board insulating material has a thickness sufficiently small for use in a circuit board. In other words, the circuit board electrically insulating material of the present invention includes a porous sheet provided with a semi-hardened resin layer on both surfaces or one surface. In the circuit board electrically insulating material, the pores of the porous sheet are hollow. Furthermore, the pores of the porous sheet of the present invention may be formed by continuous holes. Hereinafter, xe2x80x9cat least a center of the porous sheet is not impregnated with resinxe2x80x9d also is referred to as xe2x80x9cpores of the porous sheet are hollow.xe2x80x9d
Next, the circuit board of the present invention includes a desired number of electrically insulating layers and wiring layers laminated alternately, and an inner via hole for securing an electrical connection between the wiring layers by compressing and hardening a conductive paste including a conductive particle and a resin; wherein the electrically insulating layer includes a porous sheet in which a resin layer is laminated to at least one surface, and at least a central portion of the porous sheet is not impregnated with a resin; a through hole penetrating the electrically insulating layer in the thickness direction of the electrically insulating layer is filled with a conductive paste including a conductive particle and a resin, and pores that are present inside the porous sheet are filled with the laminated resin; and the average hole diameter of the pores that are present inside the porous sheet is smaller than the average particle size of the conductive particle.
Next, a first method for manufacturing a double-sided circuit board of the present invention includes: providing a through hole at a desired position of a laminate in which mold release films are formed on both surfaces of the above-mentioned circuit board electrically insulating material; filling the through hole with a conductive paste; peeling off the mold release film from the laminate in which the through hole is filled with the conductive paste; superimposing metal foils on both surfaces of the circuit board electrically insulating material from which the mold release films have been peeled off to form a laminate; heating and pressing the laminate to allow hollow pores of the porous sheet to be filled with a resin and allow the metal foils to be adhered to the porous sheet while compressing and hardening the conductive paste filled in the through hole, thereby providing an inner via hole; and forming desired circuit patterns on the metal foil.
Next, a second method for manufacturing a double-sided circuit board of the present invention includes forming a laminate either by superimposing resin sheets on both surfaces of a porous sheet that is not completely impregnated with a resin, further superimposing the mold release films on both surfaces of the laminated resin sheet, and pressing thereof; or by forming a resin layer on one surface of a mold release film, further sandwiching a porous sheet that is not impregnated with a resin by the sides of the resin layer of the mold release films provided with the resin layers; and pressing thereof; providing a through hole in a desired position of the laminate provided with the mold release films; filling the through hole with a conductive paste; peeling off the mold release films from the laminate in which the through hole has been filled with the conductive paste; superimposing the metal foils on both surfaces of the laminate from which the mold release films have been peeled off; heating and pressing the laminate to allow hollow pores of the porous sheet to be filled with a resin and allow the metal foil to be adhered to the porous sheet, and compressing and hardening the conductive paste filled in the through hole, thereby providing an inner via hole; and forming desired circuit patterns on the metal foil.
Next, a third method for manufacturing a double-sided circuit board of the present invention includes providing a through hole at a desired position of a laminate in which mold release films are formed on both surfaces of a porous sheet that is not completely impregnated with a resin; filling the through hole with a conductive paste, and peeling off the mold release films from the laminate in which the through hole has been filled with the conductive paste; sandwiching the laminate from which the mold release films have been peeled off by transfer media, the transfer media being formed on a supporting base material by filling a resin in an exposed portion of the supporting base material at the side of the wiring layer provided on the supporting base material; and heating and pressing the laminate to allow hollow pores of the porous sheet to be filled with a resin and allow the wiring layer to be adhered to the porous sheet, and compressing and hardening the conductive paste filled in the through hole, thereby providing an inner via hole; and removing the supporting base material of the transfer medium from the heated and pressed laminate.
Next, a fourth method for manufacturing a double-sided circuit board of the present invention includes forming a resin layer at the side of a wiring layer of a transfer medium in which a wiring layer is provided on a supporting base material; sandwiching both surfaces of a porous sheet that is not completely impregnated with a resin between the transfer medium on which the resin layer is formed and a mold release film provided with a resin layer to form a laminate; forming a non-through hole at a desired position of the laminate from the side of the mold release film; filling the non-thorough hole with a conductive paste; peeling off the mold release film from the laminate in which the non-through hole has been filled with the conductive paste; superimposing a metal foil on the surface of the laminate from which the mold release film has been peeled off; heating and pressing the laminate to which the metal foil is superimposed to allow hollow pores of the porous sheet to be filled with a resin and allow the metal foils and the wiring layer of the transfer medium to be attached to the porous sheet, and compressing and hardening the conductive paste filled in the non-through hole, thereby providing an inner via hole; forming desired circuit patterns on the metal foil; and removing the supporting base material from the laminate in which the circuit patterns have been formed on the metal foil.
Next, a first method for manufacturing a multilayer circuit board of the present invention repeats the following steps once or more: providing a through hole at a desired position of the laminate in which mold release films are formed on both surfaces of the above-mentioned circuit board electrically insulating materials; and filling the through hole with a conductive paste; peeling off the mold release films from the laminate in which the through hole has been filled with the conductive paste; superimposing a desired number of the circuit board electrically insulating material from which the mold release films have been peeled off and the circuit board provided with two or more of wiring layers alternately so that the circuit board electrically insulating material becomes the outermost layer, and furthermore superimposing metal foils on the surface thereof to form a laminate; heating and pressing the laminate to allow hollow pores of the porous sheet to be filled with a resin and allow the metal foil and the circuit board to be adhered to the porous sheet, and compressing and hardening the conductive paste filled in the through hole, thereby providing an inner via hole; and forming desired circuit patterns on the metal foil.
Next, a second method for manufacturing a multilayer circuit board of the present invention repeats the following steps once or more: forming a laminate either by superimposing resin sheets on both surfaces of a porous sheet that is not completely impregnated with a resin, further superimposing the mold release films on both surfaces of the laminated resin sheet, and pressing thereof; or by forming a resin layer on one surface of the mold release film, sandwiching a porous sheet that is not completely impregnated with the resin between the sides of the resin layer of the mold release film provided with the resin layer, and pressing thereof; providing a through hole at a desired position of the laminate provided with the mold release films, filling the through hole with the conductive paste, and peeling off the mold release films from the laminate in which the through hole has been filled with the conductive paste; superimposing a desired number of the laminate from which the mold release films have been peeled off and the circuit board electrically insulating material provided with two or more of the wiring layers alternately so that the circuit board electrically insulating material becomes the outermost layer, and furthermore superimposing a metal foil; heating and pressing the laminate to allow hollow pores of the porous sheet to be filled with a resin and allow the metal foil and the circuit board to be adhered to the porous sheet, and compressing and hardening the conductive paste filled in the through hole, thereby providing an inner via hole; and forming desired circuit patterns on the metal foil.
Next, a third method for manufacturing a multilayer circuit board of the present invention repeats the following steps once or more: forming a resin layer on at least one surface of the circuit board provided with two or more wiring layers; superimposing a porous sheet that is not completely impregnated with a resin at the side of the resin layer of the circuit board; and furthermore superimposing the mold release film provided with the resin layer on one surface onto the porous sheet; forming a non-through hole at a desired position of the laminate from the side of the mold release film; filling the non-through hole with the conductive paste; and peeling off the mold release film from the laminate in which the non-through hole has been filled with the conductive paste; superimposing metal foils on the surfaces of the laminate from which the mold release films have been peeled off; heating and pressing the laminate to allow hollow pores of the porous sheet to be filled with a resin and allow the metal foil and the circuit board to be adhered to the porous sheet, and compressing and hardening the conductive paste filled in the through hole, thereby providing an inner via hole; and forming desired circuit patterns on the metal foil.
As mentioned above, with the circuit board electrically insulating material, circuit boards or the methods for manufacturing a circuit board of the present invention, it is possible to make the electrically insulating layer of the circuit board which secures the electrically connection with the inner via hole including a conductive paste to be homogenized and ultra thin and the reliability of the via connection to be higher. Therefore, the present invention can provide a circuit board that is extremely suitable for high density mounting.